1. Field of the Invention
The present invention relates to a substrate processing apparatus, a method for manufacturing a semiconductor device, and a process tube.
2. Description of the Prior Art
In the fabrication of semiconductor devices such as Dynamic Random Access Memory (DRAM), substrate processing is performed by supplying gas into a process vessel where substrates are loaded. In such a substrate processing, a substrate holding unit charged with a plurality of substrates horizontally held and vertically arranged in multiple stages is loaded into the process vessel, and gas is supplied from the lower part of the process vessel and exhausted through the upper part of the process vessel. In this way, the gas is supplied to the surface of each substrate (for example, see Patent Document 1).    [Patent Document 1] Japanese Patent Publication No. 2005-243736
However, in the above-mentioned substrate processing, gas is difficult to flow through a central region of each substrate, and gas flow rates are different at the peripheral region and the central region of each substrate so that the substrate processing is not performed uniformly over the surface of the substrate. Furthermore, gas flow rates to substrates are different according to the holding position (height) of the substrate inside the substrate holding unit. Thus, the non-uniform substrate processing among the substrates, so-called loading effect, is performed occasionally.